Electromechanical drive for magnetometers

Abstract

A vibrating sample magnetometer controls an electromechanical drive using both ac and dc control and a position sensor. Absolute position sensing is used in a feedback loop to control both an alternating current (ac) drive and a direct current (dc) drive to the electromechanical drive. Improved performance and stability results.

What is claimed is:  
     
       1. In a magnetometer, a controller for in use, controlling an electromechanical drive used to generate oscillating sample motion, said electromechanical drive being coupled to move a vibrating or oscillating assembly, said controller providing ac and dc drive signals to actuate said electromechanical drive, said electromechanical drive imparting motion to said vibrating or oscillating assembly in response to said ac and dc drive signals, said controller including: 
       a position sensor that monitors the position of at least a portion of said assembly, said position sensor providing information on the relative position of the vibrating or oscillating assembly; and  
       an electronic measuring arrangement that is responsive to the output from the position sensor in a control feedback loop, said electronic measuring arrangement controlling both the ac and dc drive signals to the electromechanical drive to maintain substantially constant output from the position sensor.  
     
     
       2. The controller of  claim 1  wherein said electromechanical drive comprises a voice coil drive. 
     
     
       3. The controller of  claim 1  wherein said position sensor comprises an eddy current sensor. 
     
     
       4. The controller of  claim 1  wherein said position sensor measures absolute displacement between said vibrating or oscillating assembly and said position sensor. 
     
     
       5. The controller of  claim 1  wherein said electronic measuring arrangement includes a position feedback circuit including independent ac and dc control loops. 
     
     
       6. The controller of  claim 1  wherein said electronic measuring arrangement includes a drive source that generates a summed dc and ac current to drive the electromechanical drive and maintain substantially constant ac and dc voltage outputs from said position sensor. 
     
     
       7. The controller of  claim 1  further wherein said electromechanical drive includes an electromechanical driving transducer including at least one magnet, at least one magnetic pole plate, a flexible suspension and a coil. 
     
     
       8. The controller of  claim 1  wherein said sample comprises a sample with a magnetic moment and said vibrating or oscillating assembly includes a sample rod for retaining said sample. 
     
     
       9. The controller of  claim 1  further including a sensor target cooperating with said position sensor, said position sensor outputting a dc voltage but substantially no ac voltage when the target is stationary relative to the sensor and the relative displacement between the sensor and the target is fixed, said position sensor outputting an ac voltage proportional to an oscillation amplitude when the target is oscillating about a fixed position relative to said position sensor. 
     
     
       10. The controller of  claim 1  wherein said position sensor uses a non-contact sensing technique. 
     
     
       11. The controller of  claim 1  wherein the electronic measuring arrangement separates ac and dc components outputted from said position sensor. 
     
     
       12. The controller of  claim 1  wherein said electronic measuring arrangement maintains substantially constant ac vibration amplitude. 
     
     
       13. The controller of  claim 1  wherein said electronic measuring arrangement provides dc voltage control that maintains relative position of said vibrating or oscillating assembly with respect to said position sensor substantially constant and also maintains said vibrating or oscillating assembly position with respect to at least one magnet pull plate constant. 
     
     
       14. A method of measuring magnetic moment comprising: 
       vibrating or oscillating a sample;  
       measuring absolute position of a target mechanically coupled to said sample using a position sensor;  
       resolving an output of said position sensor into an ac component and a dc component;  
       using independent ac and dc feedback loops responsive to said ac and dc components, respectively, to control the vibration or oscillation of said sample; and  
       measuring a voltage induced in a pick-up coil in proximity to said sample in order to determine the magnetic moment of the sample.  
     
     
       15. The method as in  claim 14  wherein said vibrating or oscillating step is performed using a voice coil. 
     
     
       16. The method as in  claim 14  wherein said position sensor comprises an eddy current position sensor that measures displacement by generating and detecting eddy currents in said target. 
     
     
       17. The method as in  claim 14  further comprising generating a summed dc and ac current to drive an electromechanical assembly mechanically coupled to said sample in order to maintain substantially constant ac and dc voltage outputs from said position sensor.